The present invention relates to a rolling bearing, particularly, for use in airplanes and it relates to an improvement for the life of the bearing.
Since roll bearings are used in a severe way of undergoing cyclic shearing stresses at a high surface pressure, it is necessary to ensure a rolling contact fatigue life while withstanding the shearing stresses. Therefore, high carbon-chromium bearing steels (SUJ2) have been used so far for the material constituting bearing rings (inner and outer rings) and rolling elements of the rolling bearings to which quenching/tempering have been applied to make the Rockwell hardness to HRC 58 to 64 thereby ensuring the rolling contact fatigue life.
Further, rolling bearings for use in airplanes are used at a high temperature exceeding 200xc2x0 C. Then, for ensuring necessary hardness of the rolling bearings also at such a working temperature, semi-high speed steel series AISI M50 that causes secondary hardening by tempering at high temperature exceeding 500xc2x0 C. is used.
By the way, along with increasing speed and decreasing fuel cost in airplanes, demand for the performance of rolling bearings for use in airplanes has become severer. On the other hand, M50 used so-far-as rolling bearings for use in aircrafts leaves a problem that coarse eutectic carbides are present which leads to flaking life. Further, since air ports are often located in coastal areas in view of a problem for the site and the problem of noises, rolling bearings for use in airplanes are often used or stored in circumstance tending to cause rust by salt damages. Since fracture of the rolling bearings for use in aircrafts will lead to a fetal accident, even slight rust is often judged as limit of life. However, M50 described above also involves a problem that corrosion resistance is not enough since it contains less Cr which is most effective for the improvement of the corrosion resistance of steels.
In recent years, a carburized type M50 NiL developed with an aim of improving the toughness in the core portion of M50 has also been used but the surface characteristic is substantially identical with M50.
The present invention has been developed in order to overcome the foregoing problems and it is an object thereof to provide a rolling bearing suitable for use in airplanes which is more excellent in the rolling contact fatigue life characteristic and the corrosion resistance than AISI M50 or M50 NiL.
At present, no lubricant workable at a high temperature exceeding 250xc2x0 C. has yet been put to practical use and, therefore, the working temperature of the rolling bearing for use in airplanes is also from 200 to 250xc2x0 C., so that the present inventors have continued earnest studies on the improvement of the life characteristic and the rolling contact fatigue life, as well as on the improvement of the corrosion resistance of the rolling bearing in a temperature region from 200 to 250xc2x0 C. and have accomplished the present invention.
It has been found that steels containing a great amount of Cr are extremely effective for improving the rolling contact fatigue life characteristic at high temperature since residual austenite formed upon quenching is stable, that is, a great amount of Cr prevent decomposition of the residual austenite. Further, Cr is a most effective element for improving the corrosion resistance and it has to be added at least by 7% by weight or more in order to obtain such effects sufficiently. It is preferably added by 9% weight or more. On the other hand, for improving the rolling contact fatigue life, it is necessary that the amount of residual austenite in the surface layer is 6 vol % or more and, preferably, 10 vol % or more. However, it is necessary to previously apply tempering at a temperature higher enough than the working temperature in order to prevent dimensional change by the martensitic transformation of the residual austenite (due to tempering). Since it is aimed at a working temperature of 200xc2x0 C. or higher in the present invention, tempering has to be applied at a temperature of about 300xc2x0 C. at the lowest. Further, for the addition of Cr, even when it is added in excess of 16% by weight, its effect is saturated and increase in the amount is not only useless but also deteriorates the workability and increases the material cost to increase the cost unnecessarily, so that the upper limit is defined as 16% by weight. Further, for the amount of the residual austenite, since too much amount results in the deterioration of the hardness, 35% is preferred as the upper The rolling bearing of the present invention includes an inner ring, an outer ring and rolling elements, wherein at least one of the inner ring, the outer ring and the rolling elements comprises an alloy steel containing from 7.0% or more to 16% or less of Cr on the weight % basis, and the amount of residual austenite in the surface of at least one of the raceway surface of the bearing ring or the rolling surface of the rolling element of a completion product is 6% by volume or more.
Further, a rolling bearing of the present invention has the feature wherein the amount of residual austenite is from 10% by volume or more to 35% by volume or less.
Further, the rolling bearing of a present invention has the feature in the wherein the alloy steel contains from 0.2% by weight or more to 0.6% by weight or less of C and from 9% by weight or more to 16% by weight or less of Cr.
Further, a rolling bearing of the present invention has the feature, wherein at least one of the inner ring, the outer-ring and the rolling element is tempered at a high temperature from 350xc2x0 C. or higher to 500xc2x0 C. or lower and used at a temperature from 200xc2x0 C. or higher to a temperature lower than that for the high temperature tempering.
Further, a rolling bearing of the present invention has a feature, wherein the size of eutectic carbides present on the raceway surface or the rolling surface is 10 xcexcm or less.
Further, a rolling bearing of the present invention has a feature, wherein C and Cr contained in the alloy steel satisfy the relation: C %xe2x89xa6xe2x88x920.05Cr %+1.41.
The critical meanings in the present invention are as shown below.
Cr: 7.0% by weightxe2x89xa6Crxe2x89xa616.0% by weight
Cr is an element necessary for improving the rolling contact fatigue life at high temperature by stabilizing the residual austenite and an element most effective to provide steels with corrosion resistance. If the addition amount is less than 7% by weight, no sufficient effect to the rolling contact fatigue life characteristic and the corrosion resistance can be obtained, so that the lower limit is defined as 7% by weight. Further, it is added more preferably by 9% by weight or more in view of the corrosion resistance. However, even if it is added in excess of 16% by weight, the effect is saturated and increase in the amount not only is useless but also deteriorates the workability and increases the material cost to unnecessarily increase the cost, so that the upper limit is defined as 16% by weight.
C: 0.2% by weightxe2x89xa6Cxe2x89xa60.6% by weight
C has an effect of improving the hardness by transforming the structure martensitic after quenching and tempering thereby increasing the strength and suppressing the formation of xcex4 ferrite deleterious to the toughness, but addition of a great amount results in precipitation of coarse eutectic carbides to deteriorate the rolling contact fatigue characteristic. Accordingly, the upper limit for the addition of C is defined as 0.6% by weight. For suppressing the formation of xcex4 ferrite deleterious to the toughness, C has to be added so as to satisfy the following relation 1 and, as apparent from the equation, Ni, Co or N may be added in place of the addition of C. However, since the addition of an alloy element such as Ni, Co or N results in increase of the cost and deteriorates the workability of the material, it is desirable that C is added by 0.2% by weight or more to prevent formation of xcex4 ferrite.
Axe2x89xa71.14xc3x97Bxe2x88x9212.5xe2x80x83xe2x80x83(1)
A=Ni+Co+30C+25N
B=Cr+1.5 Mo+5V
That is, assuming Ni, Co as 0, (Mo, V, N=0) and assuming Cr as 16% by weight for the upper limit of Cr, as a preferred form, the lower limit for C in the present invention is defined, in view of the relation:
xe2x80x83A=30C+25Nxe2x89xa71.14 (Cr+1.5Mo+5V)xe2x88x9212.5, as
C%xe2x89xa7(1.14xc3x9716%xe2x88x9212.5)/30≈0.19%,
that the lower limit for C is 0.2% by weight and formation of xcex4 ferrite can be suppressed.
In a case where the Cr content is high and the C content is low, xcex4 ferrite is formed to remarkably lower the toughness but the C concentration at which xcex4 ferrite forms is lowered with addition of N. Since formation of xcex4 ferrite can be suppressed by defining the lower limit for the C concentration as: C %xe2x89xa70.04Cr %xe2x88x920.83N %xe2x88x920.39, it is desirable to satisfy the relation of the formula.
If the upper limit for the C concentration is not defined as C % is xe2x88x920.05Cr %+1.41 or less, coarse primary eutectic carbides of 20 xcexcm or more are formed to lower the acoustic characteristic and the fatigue life. Accordingly, rolling bearings excellent in them can be provided by restricting the size of the eutectic carbides to 10 xcexcm or less. Even when the relation described above is satisfied, it is often observed that the primary eutectic carbides are grown to about 5-20 xcexcm or more under the effect of a solidification rate or the like during steel making. However, in the alloy steels used for the rolling bearing of the present invention, fine secondary carbides or nitrides are precipitated thereby enabling to enhance the strength by either preventing growing of the eutectic carbides in excess of 20 xcexcm or without causing eutectic carbides at all, so long as the relation for suppressing the growing of eutectic carbides is satisfied.